. at the same time, the united states began evacuating americans from japan and u.s. officials reminded those staying behind to get out of that 50-mile danger zone around the fukushima dai-ichi plant. the reactors damaged by friday's earthquake and tsunami were bombarded today with water mr. from helicopters, police water cannons and fire trucks to try to cool them off and prevent a meltdown, but it's not at all clear if it's working. and in washington, the head of the nuclear regulatory commission said it could take weeks to get these reactors under control. bill whitaker in japan begins our coverage. >> reporter: this new video released today gives the clearest picture yet of the stricken fukushima dai-ichi nuclear power plant. >> what we're seeing is that the damage from the fires is very significant. >> reporter: today, japanese military helicopters with protective led-lined cockpits dumped water on reactor three, attempting to cool the nuclear fuel rods. but much of the water appeared to disperse in the wind. police and firefighters also brought in water cannons to douse the reactor but

after soaring radiation forces a retreat. and the u.s. tells americans to evacuate a 50-mile danger zone. i'm katie couric. also tonight, the question everyone in this country is asking: could it happen here? the u.s. has 23 nuclear reactors just like those in japan. how safe are they and we? and as the search goes on for victims of the earthquake and tsunami, an american exchange teacher is among the missing. captioning sponsored by cbs from cbs news world headquarters in new york, this is the "cbs evening news" with katie couric. >> couric: good evening, everyone. they have what could be the most dangerous job in the world, and the world is rooting for them to get it done. the nuclear power plant workers in japan trying to prevent a meltdown. radiation at the dai-ichi plant in fukushima got so high today they were forced to leave temporarily, but now they're back on the job. japan has raised the maximum radiation dose allowed for nuclear workers so they can deal with the crisis, but the head of the u.s. nuclear regulatory commission told congress today the doses those workers could be

>> couric: tonight, the u.s. uses a warplane as the allies keep up the assault on libya and qaddafi remains defiant. >> (translated): we win. we will be victorious in this historical battle. we will not surrender. >> couric: i'm katie couric. also tonight, they survived one disaster, now these japanese have been forced to take shelter against another threat-- nuclear radiation. america's nuclear problem. where to store permanently more than 145 million pounds of spent fuel rods. and college students struggling to make the grade. what some schools are doing to make sure they graduate. captioning sponsored by cbs from cbs news world headquarters in new york, this is the "cbs evening news" with katie couric. >> couric: good evening, everyone. it's four days into a u.s.-led assault on his military, and libya's moammar qaddafi has lost radar installations, tanks, and naval facilities but not his defiance. he appeared in public tonight in tripoli vowing to fight on and telling supporters he will win and will not surrendered. qaddafi's forces kept up their attacks on civilians today in a n

will lead the operation when the u.s. steps back in the days ahead, though nato is expected to play a major role. meanwhile, a u.s. air force fighter jet crashed today in eastern libya. the two men on board ejected and were rescued. a cbs news poll out tonight finds most americans are following the events in libya closely and nearly seven out of ten approve of the air strikes. mandy clark begins our coverage from the scene of that fighter jet crash. >> reporter: this is all that remains of the american f-15e that went down last night. a steady stream of people came to have a look. saleh saeed saleh, a local farmer, was eager to show us the wreckage. when it it this ground he says it sounded like a rocket exploding. he thought qaddafi's forces were on the attack. officials say the fighter jet crashed because of a mechanical error rather than any enemy fire. it landed east of benghazi which is in the heart of rebel territory. the jet's crew ejected safely. were they okay? were they injured? "the person i saw had minor injuries, just scratches" he says. one of the americans landed in a nearby

with the crisis, but the head of the u.s. nuclear regulatory commission told congress today the doses those workers could be exposed to are potentially lethal in a short period of time. it's nearly six days now since the earthquake and tsunami killed at least 4300 people and damaged the nuclear reactors. today, u.s. officials told americans within 50 miles of the plant to evacuate the area or stay indoors. that is two and a half times as wide as the danger zone established by the japanese. harry smith begins tonight's coverage of the disaster in japan. >> reporter: in a sign of how grave japan's crisis has become, the emperor, akihito, made an unprecedented television address, acknowledging that he is deeply worried, urging his subjects not to give up. it did little to calm a country increasingly distrustful, given the wave of conflicting reports and mixed messages. >> ( translated ): there is both positive and negative news. i don't know which i should believe. >> reporter: and today on capitol hill, u.s. energy secretary and nuclear expert steven chu said he, too, is baffled. >> and ther

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